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This is the 2nd post in a multipart series.
If you want to read more, see our series index

The bookmarks extension adds another feature from Fat VS to code, the ability to bookmark to a place in a document/file/code and be able to quickly navigate backwards and forwards to it. One important setting that I think you should change is bookmarks.navigateThroughAllFiles - set that to true and you can jump to any bookmark in your project, with false (the default) you can only navigate to bookmarks in the current file.

This is the 18th post in a multipart series.
If you want to read more, see our series index

Following on from the previous post we looked at operators and being able to use them yourself by implementing the relevant operator methods. The first part I want to cover in this second post is the Unary operators +, -, and !.

When I was learning this, the term unary jumped out as one I did not immediately recognise, but a quick Wikipedia read it became clear. For example, if you use a negative unary with a positive number, it becomes a negative number... It is primary school maths with a fancy name.

One thing to remember about operators is it is totally up to you what they mean, so, for example, let's start with a simple pet class to allow us to define what type of pet we have.

packageblogcode

enumclass animal {

dog,

cat

}

dataclass pet(val type: animal);

fun main(args: Array<String>){

val myPet = pet(animal.dog)

println(myPet)

}

this produces pet(type=dog)

Now, maybe in my domain, the reverse of a dog is a cat, so I can do this to make this reflect my domain:

packageblogcode

enumclass animal {

dog,

cat

}

dataclass pet(val type: animal){

operatorfun not(): pet =when(this.type){

animal.cat-> pet(animal.dog)

animal.dog-> pet(animal.cat)

}

}

fun main(args: Array<String>){

val myPet = pet(animal.dog)

println(!myPet)

}

This produces pet(type=cat)

And this is the core thing, that while a Unary has a specific purpose normally you can totally use it the way that makes sense. This is really awesome and powerful but it doesn't stop there.

Normally when we think of something like the Unary not with a boolean, it goes from true to false (or vice versa), but it remains a boolean. There is nothing stating it has to be that way:

This is the 17th post in a multipart series.
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This next post is an introduction to operators in Kotlin, not the basic the "use a plus sign to add numbers together" stuff (you read this blog, you got to be smart enough to figure that out). No, this post is about just how amazingly extensive the language is when it comes to support for allowing your classes to use them.

Adding things together

So let's start with a simple addition things together with plus I said we wouldn't do. In the following example code, we have a way to keep track of scoring events in a rugby game and I would like to add up those events to get the total score:

This obviously won't compile, you can't += an Int and my own class? Right?!

We could make this change, which is probably the better way but for my silly example, let us say this isn't ideal.

totalPoints = event.points()+ totalPoints

So to get our code to compile we just need a function named plus which has the operator keyword and whle I could call this myself, the Kotlin compiler is smart enough to now make it just work:

dataclass score(val type:scoreType){

fun points()=when(this.type){

scoreType.`try` ->5

scoreType.conversion->2

scoreType.kick->3

}

operatorfun plus(other:Int)=this.points()+ other

}

How cool is that?!

If I wanted to take it further and support say totalPoints += event then you would need to add a function to Integer which tells it how to add a score to it. Thankfully that is easy with extension functions:

operatorfunInt.plus(other:score)=this+ other.points()

Extensive

While the above is a bit silly, imagine building classes for distances, time, weights etc... being able to have a kilogram and a pound class and add them together! What makes Kotlin shine is how extensive it is, just look at this list!

Class

Operators

Method

Example Expression

Arithmetic

+, '+='

plus

first + second

Augmented Assignments

+=

plusAssign |first += second`

Unary

+

unaryPlus

+first

Increment & Decrement

++

inc

first++

Arithmetic

-, -=

minus

first - second

Augmented Assignments

-=

minusAssign

first -= second

Unary

-

unaryMinus

`-first

Increment & Decrement

--

dec

first--

Arithmetic

*, *=

times

first * second

Augmented Assignments

*=

timesAssign

first *= second

Arithmetic

/, /=

div

first / second

Augmented Assignments

/=

divAssign

first /= second

Arithmetic

%, %=

rem

first % second

Augmented Assignments

%=

remAssign

first %= second

Equality

==, !=

equals

first == second

Comparison

>, <, <=, >=

compareTo

first > second

Unary

!

not

!first

Arithmetic

..

rangeTo

first..second

In

in, !in

contains

first in second

Index Access

[, ]

get

This returns a value | first[index]

Index Access

[, ]

set

This sets a value | first[index] = second

Invoke

()

invoke

first()

I am going to go through some of these in more detail in future blog posts, but one I wanted to call out now:

Augmented Assignments vs. Plus or Minus

You might wonder why, when we just implemented plus above we got both support for + and += and the table lists += under both plus and augmented? Why both - because you may want to support just += without supporting +.

This post is one in a series of stuff formally trained programmers know – the rest of the series can be found in the series index.

The hash table, like the tree, needs a key. That key is then converted into a number using a hash function which results in the position in the array that is the data structure. So when you need to lookup an item, you pass in the key, get the same hash value and that points to the same place in memory so you get an O(1) lookup performance.

This is better than a pure array, where you cannot do a lookup so you have to search through giving an O(n) performance and better than a tree which also needs a search, so it ends at O(log n).

A hash table would be implemented with an array, which means that inserting is O(1) - unless the array is full then it is O(n), so this is a pretty good level of performance. An important note in comparing, O(1) when adding to the end of the array and O(1) with a hash table is that the hash table has additional computation so that while they have the same complexity it is still slower to use a hash table; this is why big O notation is useful for understanding it has limits on how it can help us choose what to use.

A hash table, in concept, is easy, but in practice, it is another thing altogether. First, you need a hash algorithm that ends up giving a value inside the bounds of the array - this is often done using modulus. For example, if we get a value of 14213 but our array is only 8 in size we can do 14213 % 8 to get its position, 5.

This converting of numbers brings a new problem, collisions; namely, we are taking a large number of possible numbers and converting them to a smaller number, so what happens when we get say 85? It also has a modulus of 5! Two items can't live in the same location. There is no single solution to collisions, there are many.

One option uses linked lists in the items, so if there is a collision then you can store it at the same index and then you only have to search the, hopefully, very limited set in the collisions list. Others use offsets to shift collided items around the existing array. For this series though, a complete analysis of the options is beyond the scope.

Implementations

This is the 15th post in a multipart series.
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Our last post covered a lot of the awesome extensions to collections in Kotlin and as someone who comes from .NET and Java I think about writing Lambdas in a specific way.

val numbers = listOf(1,5,6,8,10)

val evens = numbers.filter({ num -> num %2==0})

println(evens)

In .NET/Java we need a range variable, in the example above it is num which refers to the item in the collection we are working on. Kotlin knows you will always have a range variable, so why not simplify it and make it have a common name it:

val numbers = listOf(1,5,6,8,10)

val evens = numbers.filter({ it %2==0})

println(evens)

Here we got ride of the num -> start of the lambda and we can just refer to the automatically created range variable it.

This is the 15th post in a multipart series.
If you want to read more, see our series index

The Koans for Kotlin, are broken into sections and our earlier posts have covered the first section; this post is going to cover the entire second section. The reasons we are not breaking this down as we did before is because the common theme here is working the kotlin.collections and in particular the wealth of extension methods that exist.

Any returns true if it finds one any match to the provided expression. All requires all items in the collection to match the provided expression. Count provides the size of the collection. FirstOrNull returns the first item in a collection which matches the expression or, if nothing does, it returns null

Any, all and count work the same as in .NET. Count should be avoided when size exists, since Count could cause it to be on O(n) operation to determine the sized, though it depends on the collection implementation. firstOrNull is similar to .NETs firstOrDefault except, it returns null if there is no match where .NET might return null (only for reference types, like classes) and what the default is for value types (0 for int, false for boolean etc...)

This is the 14th post in a multipart series.
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The backtick in Kotlin is meant to allow Kotlin to use keywords or, since Kotlin is meant to interop with Java, allow Kotlin to call Java functions that might conflict with Kotlin; for example, this won't work

valclass="school"

println(class)

If we wrap class in backticks (as it is a keyword) then it works just fine:

val `class` ="school"

println(`class`)

This is nice... however it can be used for function names too, for example with tests we might use underscores to make the name verbose:

This is the 12th post in a multipart series.
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We are going to take a short break from the Koans and look at a cool trick I learnt today. Previously, we learnt about the safe null operator but that only helps when calling functions or properties of objects...

In the above example, we have the name String which could be null but safe null operator (this needs a better name) can't help here... so let us look at what Kotlin calls the Elvis operator ?: and what it gives us: